WO2019063403A1 - Procédé et dispositif de mesure sans contact d'une distance à une surface ou d'une distance entre deux surfaces - Google Patents

Procédé et dispositif de mesure sans contact d'une distance à une surface ou d'une distance entre deux surfaces Download PDF

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Publication number
WO2019063403A1
WO2019063403A1 PCT/EP2018/075441 EP2018075441W WO2019063403A1 WO 2019063403 A1 WO2019063403 A1 WO 2019063403A1 EP 2018075441 W EP2018075441 W EP 2018075441W WO 2019063403 A1 WO2019063403 A1 WO 2019063403A1
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Prior art keywords
light
measuring
calibration
calibration light
measurement
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PCT/EP2018/075441
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German (de)
English (en)
Inventor
Christoph Dietz
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Precitec Optronik Gmbh
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Priority to CN201880062730.0A priority Critical patent/CN111373301B/zh
Priority to JP2020518006A priority patent/JP7410853B2/ja
Publication of WO2019063403A1 publication Critical patent/WO2019063403A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
    • G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material
    • G01B11/0608—Height gauges
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
    • G01B9/0207—Error reduction by correction of the measurement signal based on independently determined error sources, e.g. using a reference interferometer
    • G01B9/02072—Error reduction by correction of the measurement signal based on independently determined error sources, e.g. using a reference interferometer by calibration or testing of interferometer
    • G01B9/02074—Error reduction by correction of the measurement signal based on independently determined error sources, e.g. using a reference interferometer by calibration or testing of interferometer of the detector
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/0209—Low-coherence interferometers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B2210/00—Aspects not specifically covered by any group under G01B, e.g. of wheel alignment, caliper-like sensors
    • G01B2210/50—Using chromatic effects to achieve wavelength-dependent depth resolution

Definitions

  • the invention relates to a measuring device and a method for non-contact measurement of a distance to a surface or a distance between two surfaces, wherein polychromatic measuring light directed to a measuring object and the measured light reflected from the measuring object is spectrally analyzed.
  • the invention relates to the problem of reducing the accuracy of measurement by thermal effects.
  • the problem often arises of measuring the distance between a reference point and the surface of a solid or liquid object to be measured.
  • the distance is measured only at one or a few points on the surface or at a multiplicity of closely spaced points.
  • a typological one-dimensional or two-dimensional height profile of the surface of the measurement object can be derived from the distance measurement values.
  • unevenness in precisely machined surfaces can be detected or roughness parameters can be determined.
  • the term "layer” refers not only to a layer made of a material which is carried or fastened by a solid body, but also comparatively thin solid structures which do not require supports. Examples of these are panes made of glass or a semiconductor material, or walls of bottles or similar objects.
  • optical measuring principles For contactless measurement of distances, in addition to capacitive or other electrical measuring principles, above all optical measuring principles are used, since this allows a particularly high measuring accuracy to be achieved.
  • polychromatic measuring light is directed onto the measuring object with the aid of an optical measuring head.
  • the measuring light reflected from the surface of the test object is picked up by the measuring head and fed to a spectrograph, which spectrally analyzes the reflected measuring light. From the spectral composition of the measurement light can be deduced the distance to the surface of the measurement object. Since each optical interface between two different refractive-index media reflects a part of the incident light, in this way the distances to a plurality of surfaces arranged in succession in the propagation direction of the measuring light can be determined. The only condition for this is that the optical media passing through the measuring light are sufficiently transparent for the measuring light used.
  • a first type of measuring device which makes use of this measuring principle, the concept of chromatic confocal measurement is used.
  • This type of measuring device has a measuring head, which contains a chromatically uncorrected optics, which focuses the measuring light on the surface of the measuring object.
  • the optics which may include a lens made of a glass with a high dispersion and / or a diffractive optical element, the spectral components of the measuring light are focused in different focal planes.
  • a confocal arranged diaphragm ensures that only the spectral component of the measuring light whose focal plane is located exactly on the surface of the measurement object reaches the spectrograph and can be spectrally analyzed there.
  • the spectrograph includes a grating or other dispersive optical element and a detector having a plurality of photosensitive cells. Because every photosensitive cell is a very narrow one Wavelength range is assigned, the individual cells of the detector can be assigned a distance value immediately. The association between the distance values and the cells can be determined by means of a calibration, as described in DE 10 2004 049 541 A1.
  • the concept of optical interference is used. Measuring light reflected by the measuring object interferes with measuring light that has been reflected in a reference arm. Due to the interference, the reflected measuring light is spectrally modulated, whereby the sought distance value can be derived from the modulation frequency. For this purpose, measuring light which has been reflected by the measuring object and has interfered with measuring light reflected in the reference arm has been spectrally detected in a spectrograph and subjected to an inverse Fourier transformation.
  • the two types of optical measuring devices described above are frequently used in production environments for quality assurance. In the production environment, however, the ambient temperatures can fluctuate greatly. Typically, the measuring devices are therefore specified as operable for a temperature range between +5 ° C and +60 ° C.
  • the spectrograph is used to determine the spectral composition of a measuring light component which has been reflected on an optical surface in the measuring head. It is assumed that the distance of this surface to the end of an optical fiber, into which the reflected measuring light is coupled, depends on the temperature in the measuring head. In this way, the temperature can be measured indirectly over the distance from the optical surface in the measuring head, so that the influence of the temperature can be taken into account in subsequent measurements on the measuring object.
  • temperature-dependent effects in the measuring head are also taken into account in the measuring device known from DE 10 2015 1 18 069 A1.
  • the measuring light is directed in an additional measuring arm on a stepped reflective surface, wherein the height of the step along the optical axis is known exactly. If the measured distance value changes between the two surfaces, this effect is assigned to a changed temperature and used to determine a correction factor. For subsequent measurements, the measured distance values are then multiplied by the correction factor.
  • the US 9,541, 376 B2 deals with the problem of how temperature changes in the spectrograph affect the measurement accuracy in a working on the chromatic-confocal measuring principle distance measuring device.
  • the detector of the spectrograph not only either the + 1st or -1. Diffraction order, but both the + 1 and the -1.
  • the distance information is derived from the distance between the photosensitive cells of the detector where the maximum of the two orders of diffraction occurs. If the position of the grating and / or the detector changes as a result of a temperature change, this frequently leads to the fact that the maxima of the diffraction orders shift by the same amount, so that the distance between the maxima remains constant. Since only this distance is included in the further evaluation, temperature changes do not affect the measurement accuracy.
  • a disadvantage of this known approach is that two diffraction orders must be evaluated by the detector. With the same number of available photosensitive cells thereby reduce the spectral resolution and thus the measurement accuracy.
  • the object of the invention is to improve a measuring device and a method for non-contact measurement of a distance to a surface or a distance between two surfaces so that temperature changes in the spectrometer or at least to a much lesser extent than previously affect the measurement accuracy. Losses in the spectral resolution and thus the accuracy of measurement, as they must be taken into account in known solutions, should not be taken into account.
  • the above object is achieved by a measuring apparatus for non-contact measurement of a distance to a surface or a distance between two surfaces having a measuring light source configured to generate polychromatic measuring light.
  • the measuring device also has an optical measuring head, which is set up to direct the measuring light generated by the measuring light source to a measuring object and to record measuring light reflected by the measuring object.
  • the measuring apparatus comprises a spectrograph, which is set up to spectrally analyze measuring light reflected by the measuring light and picked up by the optical measuring head, the spectrograph being a dispersive optical element and a detector having a plurality of photosensitive cells.
  • An evaluation device of the measuring device is set up to calculate distance values from measurement signals of at least part of the photosensitive cells.
  • the measuring device has a calibration light source which is set up to generate calibration light having a known spectral composition.
  • the calibration light can be directed through the dispersive optical element to the detector without having been previously reflected in an optical path leading to the measurement object.
  • the evaluation device is set up to derive correction values from changes in a spectrum generated by the calibration light on at least some of the photosensitive cells of the detector, with which a predetermined association between the at least one part of the photosensitive cells on the one hand and wavelengths or wavelength-derived quantities on the other hand is modified.
  • the invention is based on the recognition that temperature changes in the spectrograph represent a significant cause of measurement inaccuracies. It should be taken into account that different temperatures can prevail at the measuring head and in the spectrograph. In the immediate vicinity of the spectrograph is, for example, often the evaluation, which usually includes a variety of electronic components. The heat loss generated by this can lead to higher temperatures in the spectrograph than in the measuring head.
  • calibration light with a known spectral composition is directed onto the detector through the dispersive optical element of the spectrograph.
  • a clear relationship between the spectrum of the calibration light on the one hand and the photosensitive cells of the detector on the other hand can be produced. If, as a result of a temperature change, the position of the photosensitive cells changes, the intensity maxima in the spectrum of the calibration light appear on other photosensitive cells.
  • this offset is taken into account directly in the evaluation of the subsequent measurements on a measurement object by the predetermined assignment between the light-sensitive cells on the one hand and wavelengths or wavelengths derived from the wavelengths, on the other hand.
  • a modified assignment may, for example, be such that a specific photosensitive cell no longer corresponds to an originally predetermined wavelength but to a modified wavelength. Since the required distance information is coded in the spectrum of the measurement light, the correct acquisition of the spectrum, which is the result of the modified assignment, automatically supplies temperature-independent measured values for the distances. This immediate type of calibration is advantageous over those approaches where the influence of temperature changes is derived from distance measurements.
  • the calibration light Since the calibration light is supplied to the spectrograph without first being reflected in an optical path leading to the measurement object, the calibration light exclusively records the influence of temperature changes in the spectrograph.
  • the possibility afforded by the invention of being able to determine the influence of temperature changes in the spectrograph independently of the influence of any temperature changes in the measuring head is advantageous for several reasons. If combined temperature changes in the spectrograph and in the measuring head are detected, as is the case with the abovementioned EP 2 149 028 B1, only the superposition of the two effects can be detected. Since both effects usually require different corrective measures during the evaluation, no optimal correction can be carried out with combined detection of the temperature changes.
  • a separate detection of the temperature changes is also advantageous in view of a modular design of the measuring device. Especially when the effects caused by temperature changes in the spectrometer dominate over the effects in the measuring head, it is advantageous if it is possible to carry out corrections independently of whether the measuring head has means for temperature detection or not.
  • the Measuring device according to the invention thus requires no special measuring heads, but can be operated with any measuring heads and is therefore universally applicable.
  • the light dividing device may be, for example, a beam splitter cube or a fiber coupler.
  • the evaluation device is set up to derive the correction values from changes in the position of intensity patterns which are generated on the at least part of the photosensitive cells of the detector by the calibration light.
  • intensity patterns usually consist of a sequence of local intensity maxima and intensity minima.
  • an intensity pattern consists of a single (local) intensity maximum or minimum.
  • the calibration according to the invention can be carried out most simply if the calibration light has a temporally stable and temperature-independent spectral composition.
  • the calibration light source should for this purpose have the property to produce regardless of the ambient temperature calibration light with a constant spectral composition.
  • the calibration light source is a narrow-band light source, for example a laser diode.
  • the calibration light source comprises a (possibly broadband) light source and a temperature-stable monochromator, for example a Fabry-Perot interferometer.
  • a particularly simple way to generate calibration light with a stable and temperature-independent spectral composition is to use a calibration light source having a broadband light source and an array of reflective surfaces that spectrally modulate the intensity of the calibration light by generating interferences.
  • the calibration light does not have a single intensity maximum at a particular wavelength, but a relatively broad spectrum, but because of its modulation has several local intensity maxima.
  • the detector of the spectrograph detects not only the position of a single intensity maximum, but several intensity maxima.
  • Such an arrangement may be formed, for example, as a plane-parallel plate made of an optically transparent material. If the plate consists of an athermal glass, changes in thickness due to temperature fluctuations are compensated by opposing changes in refractive index, so that the optical thickness of the plate and thus the modulation frequency remain constant.
  • a glass plate it is also possible to use an air gap of thickness d for spectral modulation.
  • This air gap may be formed, for example, between a first, transparent and partially reflecting on the bottom plate and a second reflective plate.
  • the distance between the plates is adjusted by a spacer made of a material of low thermal expansion (eg quartz glass or Zerodur).
  • a spacer made of a material of low thermal expansion (eg quartz glass or Zerodur).
  • the calibration can be performed simultaneously with a distance measurement.
  • the spectrum of the calibration light may be shorter than the spectrum of the measurement light.
  • the calibration must be performed at intervals between distance measurements, otherwise the distance measurement would be corrupted by the calibration light.
  • An overlap here also means the special case of identity (ie a complete overlap).
  • the calibration light is so distinguished by the dis- can be directed to the detector persistent optical element that calibration light with a wavelength falls on photosensitive cells on which no reflected measuring light with the same wavelength can fall.
  • this also applies vice versa, ie reflected measuring light can not fall on photosensitive cells, can fall on the calibration light of the same wavelength.
  • This can be achieved, for example, by the calibration light and the measuring light being polarized differently and by means of polarizing filters on the light-sensitive cells ensuring that calibration light and reflected measuring light can not fall on the same cells.
  • the light-sensitive cells may then comprise, for example, first cells, on which only the calibration light can fall and which are arranged along a first row, and second cells, on which only the reflected measuring light can fall and which is arranged along a second row, which is parallel to the first line passes.
  • the term calibration light source is to be understood broadly in the present context. In particular, it is not necessary for the calibration light and the measuring light to be produced by different optical components. In one embodiment, the calibration light source and the measurement light source use the same optical component to generate the light.
  • the calibration light source then has, for example, a beam splitter, which supplies a part of the light generated by the optical component as measuring light to the measuring head and supplies another part of the light to a monochromator, which generates the calibration light from the light by spectral filtering. Alternatively, this portion of the light may also be directed to a plane-parallel plate to produce a modulated calibration light spectrum. In this way, the calibration light source does not need its own optical component. However, it must then be ensured that during the calibration, no measurement object is located in the beam path or the optical path leading to the measurement object is dimmed, so that during measurement light in an optical path leading to the measurement object is reflected and reaches the detector.
  • two calibration light sources are provided, which are set up to produce calibration light with different spectral composition.
  • the calibration light generated by the two calibration light sources is then directed simultaneously through the dispersive optical element onto the detector without having previously been reflected in an optical path leading to the measurement object.
  • temperature-induced changes in position of the photosensitive cells can be detected, which can not be described as a uniform spatially independent offset ioffset).
  • first and second calibration lights of different spectral composition are used, a linear dependence of the offset on the wavelength can be detected and taken into account in the modification of the association between the photosensitive cells and wavelengths or quantities derived therefrom.
  • the spectra of the calibration light generated by the two calibration light sources should not overlap.
  • the spectrum of the measuring light lies between the spectra of the first and second calibration light generated by the two calibration light sources.
  • Another possibility for producing measurement signals generated by the calibration light at different locations on the detector is to use a diffraction grating as the dispersive optical element and to select the spectrum of the calibration light such that two different diffraction orders of the calibration light can be detected by the detector. Again, there are the locations where the calibration light is incident on the detector, ideally at its opposite ends and outside the intervening area reserved for the measurement light.
  • the invention also provides a method for the non-contact measurement of a distance to a surface or a distance between two surfaces, comprising the following steps: a) polychromatic measuring light is generated; b) the measuring light is directed onto a measuring object with the aid of an optical measuring head and measuring light which has been reflected by the measuring object is picked up by the optical measuring head; c) the measurement light reflected from the measurement object and picked up by the optical measurement head is spectrally analyzed in a spectrograph having a dispersive optical element and a detector with a plurality of photosensitive cells; d) distance values are calculated from measurement signals of at least a portion of the photosensitive cells, whereby a predetermined association between the at least one part of the photosensitive cells on the one hand and wavelengths or quantities derived from wavelengths on the other hand is used; e) generating calibration light having a known spectral composition; f) the calibration light is directed through the dispersive optical element onto the detector without the calibration light having previously been reflected in an optical path leading to
  • Step h) modified assignment is used.
  • the calibration light has a time-stable and temperature-independent spectral composition.
  • the calibration light can be generated in step e) by a temperature-stable monochromator, which is illuminated by a broadband light source.
  • a calibration light source for generating the calibration light may comprise a broadband light source and a plane-parallel plate made of an optically transparent material, which spectrally modulates the intensity of the calibration light by generating interferences.
  • the calibration light and the measurement light preferably have spectra that do not overlap, wherein the spectrum of the calibration light may be, in particular, shorter wavelength than the spectrum of the measurement light. In this case, it is possible to direct the calibration light onto the detector simultaneously with the measurement light and to analyze it spectrally.
  • the calibration light and the measurement light have spectra that overlap, the calibration light should not be directed onto the detector at the same time as the measurement light.
  • a part of the measuring light generated by the measuring light source can be branched off. By spectral filtering the calibration light is then generated from the branched part of the measuring light.
  • first and second calibration lights of different spectral composition are generated and simultaneously directed through the dispersive optical element to the detector without having been previously reflected in an optical path leading to the measurement object.
  • the spectra of the first and the second calibration light preferably do not overlap.
  • the spectrum of the calibration light can be selected such that two different diffraction orders of the calibration light can be detected by the detector.
  • the calibration with the help of the calibration light can be carried out at longer intervals, since the temperature usually changes only comparatively slowly. However, it is also possible for each measurement to be performed simultaneously (eg in the case of different spectra) or shortly thereafter or before to carry out a calibration with the aid of the calibration light. In this case, not only steps a) to d) but all the preceding steps a) to h) are repeated in step i).
  • the spectral broadband and z. may contain several colors, or has a plurality of narrow-band spectral components, as they are produced for example by a comb filter.
  • a dispersive optical element is understood to mean an optical element in which an optical property standing in the foreground for the function, e.g. the refractive index or a diffraction angle, shows a pronounced dispersion and the dispersion is desired for the function.
  • a normal lens made of glass thus - although the refractive power is slightly wavelength-dependent - is not a dispersive optical element.
  • dispersion prisms or diffraction gratings which show a strong dispersion and are designed to break light of different wavelengths to different extents or to bow.
  • Figure 1 is a schematic representation of a device for measuring
  • Figures 2a and 2b is a schematic representation of how pixels of a detector contained in the spectrograph ent detect an intensity maximum in the spectrum, before and after a temperature increase;
  • FIGS. 3a and 3b show a schematic representation of how pixels of a detector contained in the spectrograph capture two intensity maxima in the spectrum, namely before and after a temperature increase;
  • a first embodiment of the invention in a similar to the figure 1 schematic representation that makes use of the principle of chroma table confocal measurement makes ⁇ a second embodiment of the invention, which also makes use of the principle of chromatic-confocal measurement and in which the calibration light source a Monochromator contains ⁇ a variant for a calibration light source containing two different monochromators; a variant for a calibration light source containing a glass plate for generating spectrally modulated calibration light; the spectrum of calibration light produced by the glass plate; a variant for a calibration light source, in which the calibration light focussed on the glass plate falls; a variant for a calibration light source, in which the calibration light passes through a glass plate in transmission; a third embodiment of the invention, which also uses the Prin zip the chromatic-confocal measurement makes use and in
  • FIG. 13 shows a fifth exemplary embodiment of the invention, in which the measuring device makes use of the interferometric measuring principle
  • FIG. 14 shows a sixth exemplary embodiment of the invention in which the calibration light and measuring light can not fall on the same pixels of the detector.
  • a measuring light source 1 1 generates polychromatic measuring light 12, which is directed to a measuring object 18 via a light dividing device 14, which may, for example, be a beam splitter cube, and via a measuring head 16.
  • a light dividing device 14 which may, for example, be a beam splitter cube
  • the part of the measurement light 12 which is reflected by a surface 19 of the measurement object 18 is indicated by black arrows and is provided with the reference number 12 '.
  • the reflected measuring light 12 ' is picked up by the measuring head 16 and directed by the light dividing device 14 onto a spectrograph 20.
  • the spectrograph 20 includes a dispersive optical element 22, which may be, for example, a diffraction grating or a dispersion prism.
  • the spectrograph 20 includes a detector 24 that includes a plurality of photosensitive cells 26.
  • the photosensitive cells 26 are arranged along a straight or curved line and are referred to below as pixels.
  • the signals generated by the pixels are evaluated by an evaluation device 28 in order to calculate therefrom a distance value to the surface 19.
  • the reflected measurement light 12 ' is deflected by the dispersive optical element 22, the deflection angle depending on the wavelength of the reflected measurement light 12'. If the reflected measuring light 12 'is monochromatic, as is the case with chromatic-confocal measuring devices, then the reflected measuring light falls only on one or a few pixels 26 of the detector 24, as indicated in FIG. 1 by a blackened pixel 26' is. In measuring devices in which the reflected measuring light 12 'interferes with measuring light which was previously reflected in a reference arm (not shown), a broad spectrum is obtained on the detector 24 which is spectrally modulated.
  • the detector 24 then detects a plurality of intensity maxima, wherein each distance between the measuring object 18 and the measuring head 16 is assigned a modulation frequency.
  • the desired distance value can be calculated from the signal generated by the detector 24, as is known per se in the prior art.
  • the measuring device 10 operates on the principle of chromatic-confocal measurement. All considerations apply, mutatis mutandis, but also for interferometer measuring devices.
  • FIG. 2a shows a plurality of pixels 26 of the detector 24 and, above them, an intensity distribution which is generated by the dispersive optical element 22 from the reflected measuring light 12 '. It can be seen that the spectral intensity distribution over several pixels 26 is blurred. Each pixel 26 generates an electrical signal, which preferably depends linearly or according to a more complicated characteristic on the intensity of the incident light. By comparing the output signals of the pixels 26, it can be easily determined on which pixel 26 the highest intensity is reached. In FIG. 2 a, this pixel is marked black and designated by 26 '. This pixel can be assigned a specific wavelength.
  • a distance value can be derived directly from the information on which pixel the highest intensity occurred.
  • the association between pixels p and wavelengths ⁇ may take the form of an allocation table, for example, as shown below:
  • Each pixel pi is assigned a specific wavelength ⁇ .
  • the use of an allocation table is particularly useful if the relationship between the pixels and the wavelengths can not be specified by a simple equation.
  • the allocation table is determined by the manufacturer of the measuring device by a calibration, is determined with a tunable calibration light source, to which pixel light of a wavelength ⁇ from the dispersive optical element 22 is directed. This calibration is performed at a precisely specified temperature.
  • the assignment table between pixels and wavelengths is modified according to the invention. If, for example, the displacement ⁇ is pixels for all pixels, the following correction calculation can be carried out:
  • a k0 rr (pi) is the corrected wavelength for a signal at pixel pi and ⁇ ( ⁇ is the value resulting from the original mapping table.
  • FIG. 4 shows, in a representation similar to FIG. 1, a first exemplary embodiment of a measuring device 10 according to the invention. This has the same design as in FIG. 1 As shown and known in the prior art measuring device on a measuring light source 1 1, a light dividing device 14, a measuring head 16 and a spectrograph 20, which includes a dispersive optical element 22 and a detector 24 with pixels 26.
  • the measuring device 10 has a calibration light source 30, which is designed in the illustrated embodiment to generate calibration light.
  • the calibration light consists of two separate and narrowband spectral components, which are referred to below as the first and second calibration light.
  • the first calibration light 32a with the wavelength Aa is indicated by arrows 32a and the second calibration light 32b with the wavelength A b by arrows 32b.
  • the first and second calibration light 32a, 32b is directed by the light splitting device 14 through the dispersive optical element 22 onto the detector 24, without first being reflected in an optical path leading to the measurement object 18.
  • the dispersive optical element 22 deflects the calibration light as a function of the wavelength, so that the intensity maximum of the first calibration light 32a falls on a first pixel 26a and an intensity maximum of the second calibration light 32b on a second pixel 26b.
  • the calibration light source 30 is designed such that the first and second calibration light 32a, 32b have a time-stable and temperature-independent spectral composition.
  • the wavelengths Aa, Ab are therefore constant at all temperatures occurring during normal operation.
  • the corrected pixel number p k0 rr can be calculated by the following linear equation:
  • Pkorr Pmess A ⁇ P (A a) - "a (Ap (A b) - Ap (A a)) ⁇ ''
  • Ap (Aa) and Ap (A b ) of the offset measured by means of the calibration light 32a, 32b for the two wavelengths Aa, A b and p m ess is the pixel at which the highest intensity was measured.
  • the corrected distance value can then be easily calculated from the corrected pixel position in the chromatic-confocal distance measurement.
  • one wavelength calibration light can be used and made to fall on the detector 26 at two different locations.
  • a diffraction grating is used as the dispersive optical element 22, it can be designed so that both the + 1 and the -1. Diffraction order of the calibration light from the detector 26 can be detected. In this way one also obtains two widely spaced locations on the detector 24, at which calibration light of known wavelength impinges. Ideally, the pixels to which the measurement light 12 falls during the distance measurement are located between these locations.
  • the calibration light source 30 only generates single-wavelength calibration light, then no scaling errors can be determined. In measuring devices 10, where scaling errors do not occur or are negligibly small, it is therefore sufficient to generate calibration light with only one wavelength.
  • FIG. 5 shows a schematic illustration of a second exemplary embodiment of a measuring device 10 operating on the chromatic-confocal measuring principle.
  • the measuring light source 11 here consists of an LED 34 which generates polychromatic light with wavelengths between approximately 500 nm and 700 nm.
  • the measuring light 12 is coupled by a converging lens 36 in an optical fiber 38 and passes through a light splitting device, which is designed as a fiber coupler 40 to the measuring head 16. There, the measuring light 12 exits from an optical fiber 39 and is one of two lenses 42nd , 44 existing lens, which is not chromatically corrected, directed to the measuring object 18.
  • the emerging measuring light 12 is focused in different wave planes, as indicated in FIG. 5 for three different wavelengths.
  • the measuring light 12 reflected on the surface 19 of the measuring object 18 passes back into the optical fiber 39 via the measuring head 18 and is fed via the fiber coupler 40 to a further optical fiber 41, which leads to the spectrograph 20.
  • the calibration light source 30 has a broadband LED 46, which generates light in the blue-violet spectral range.
  • the calibration light 32 is collimated by a converging lens 48 and passes through a monochromator 50, which filters out a narrow frequency band from the spectrum of the calibration light 32.
  • the now monochromatic calibration light 32 is coupled by a converging lens 52 into an optical fiber 54, which is connected to the fiber coupler 40 such that the calibration light is guided via the optical fiber 41 to the spectrograph 20.
  • the calibration light 32 and also the measurement light 12 are collimated by a converging lens 55 and directed onto a dispersive optical element, which is designed as a reflection grating 56.
  • the light reflected and diffracted thereon is directed via a further converging lens 57 to the detector 24 with the pixels 26, which is connected to the evaluation device 28.
  • the calibration light source 30 generates only single-wavelength calibration light so that no wavelength-dependent pixel offset can be detected.
  • FIG. 6 shows a calibration light source 30 according to a variant, in which a monochromator 50 consisting of two subelements is arranged behind the lens 48 in the collimated beam path.
  • the two sub-elements are arranged in the beam path such that approximately half of the calibration light 32 passes through the sub-element 50a and the other half through the sub-element 50b.
  • Both partial elements 50a, 50b have a different filtering effect, so that in the exemplary embodiment shown in FIG. 6 first and second calibration light 32a, 32b having different wavelengths are generated.
  • the subelements 50a, 50b may be, for example, Farbry-Perot interferometers.
  • One possible design of such an interferometer comprises a plate with plane-parallel and partially reflecting coated surfaces. The wavelengths of light that the interferometer can penetrate depends on the thickness of the plate. With different plate thicknesses, a different spectral filtering can thus be achieved.
  • the calibration light does not consist of one or two monochromatic portions, but is polychromatic as well as the measurement light 12.
  • the spectrum I (p) of the calibration light is spectrally modulated, as FIG. 8 shows. If the intensity maxima are far enough apart, they can be resolved with sufficient accuracy by the detector 24 and assigned to individual pixels.
  • the broadband calibration light generated by the LED 46 is directed via a diaphragm 51, a beam splitter cube 53 and a condenser lens 59 onto a transparent plate 58 whose rear side 60 is complete and its front side 62 partially the calibration light 32 reflected.
  • the spectral modulation shown in FIG. 8 arises, the variable part of which is proportional to 4 ⁇ ⁇ ( ⁇ ) (equation 4) cos * d
  • n denotes the refractive index and d the thickness of the plate 58.
  • the plate 58 is therefore made of a glass whose thermal expansion coefficient and refractive index changes are negligibly small at the temperatures usually occurring. Then, the modulation frequency and thus the position of the intensity maxima on the detector 26 remain constant over a wide temperature range, even if the emission spectrum of the LED 46 changes with temperature changes.
  • the plate 58 Another possibility is to use an athermal glass for the plate 58, in which a thermally induced increase in the geometric thickness is at least substantially compensated by an opposite reduction in the refractive index.
  • the optical thickness of the plate 58 which is defined as the product of geometric thickness and refractive index and determines the modulation frequency, remains constant even with temperature changes with high accuracy.
  • athermal glasses are z. B. N-PK51 and N-FK51 A from Schott.
  • two locations on the detector 26 can be illuminated, between which there are the pixels which are illuminated by the measuring light.
  • two diffraction orders of the calibration light are detected by the detector 26, as described above in connection with the first exemplary embodiment.
  • a dashed line shows a spectral filter 64, which allows only wavelengths smaller than a cut-off wavelength to pass.
  • the cut-off wavelength is shorter than the smallest wavelength of the measuring light 12. In this way it is ensured that no calibration light can reach the detector 24, which is located in the spectrum of the measuring light 12. Consequently, the calibration light 32 can not affect the actual distance measurement.
  • Such a spectral filter 64 is useful when the spectrum of the LED 46 of the calibration light source 30 partially overlaps with the spectrum of the LED 34 of the measuring light source 11. With non-overlapping spectra, the spectral filter 64 can be dispensed with.
  • FIG. 9 shows a further variant for a calibration light source 30, which differs from the variant shown in FIG. 7 only in that the calibration light 32 impinges on the plate 58 not as a collimated, but as a focused beam.
  • different optical elements generate the measurement light and the calibration light. In this way, it is particularly easy to ensure that the spectra do not overlap and the calibration can also be performed during a measurement.
  • FIG. 11 shows an exemplary embodiment of such a construction, in which the calibration light source 30 only consists of a converging lens and the plate 58 of the exemplary embodiment shown in FIG.
  • Measuring light 12 generated by the measuring light source 11 is fed to the calibration light source 30 via the fiber coupler 40 and directed onto the plate 58.
  • the calibration light is a spectral modulation impressed, as has been explained above with reference to Figure 7.
  • the spectrally modulated calibration light then passes through the optical fiber 41 into the spectrograph 20 as in the exemplary embodiment shown in FIG. 5.
  • the waiver of the LED 46 in the calibration light source simplifies the construction of the calibration light source 30.
  • An additional light-generating optical element can also be dispensed with if a part of the measuring light is branched off and supplied to the spectrograph with the aid of a spectral filter.
  • the calibration can be performed simultaneously with the distance measurement. In this variant, however, less bandwidth is available for the distance measurement, which reduces the measuring range for the distance measurement.
  • FIG. 12 shows a further exemplary embodiment of a measuring device 10 according to the invention, which substantially corresponds to the variant shown in FIG.
  • the measuring light source 1 1, the calibration light source 30, the measuring head 16 and the spectrograph 20, the measuring light 12 and the calibration light 32 does not propagate in optical fibers, but in the free space.
  • the fiber coupler of the embodiment shown in Figure 5 is therefore replaced by a beam splitter cube 40 '.
  • the beam path in the spectrograph 20 is also folded such that the measurement light 12 and the calibration light 32 pass through the same lens 56 both before and after the diffraction at the reflection grating 56.
  • the measuring light 12 emerges from an exit window 70 of a measuring light source, which is otherwise not shown, and is directed onto the measuring head 16 via two diaphragms 72, 74 via the beam splitter cube 40 '.
  • FIG. 13 shows an exemplary embodiment of such a measuring device 10. This corresponds largely to the exemplary embodiment shown in FIG. 5, with the difference that the objective contained in the measuring head 16 is chromatically corrected and additionally a reference arm 80 having an end-side mirror 82 to a fiber coupler 84 connected.
  • measurement light 12 generated by the measurement light source 11 is reflected at a mirror 86 and interferes in the fiber coupler 84 with the measurement light 12 'reflected on the surface 19 of the measurement object 18.
  • the interference is detected by the spectrograph 20 and generates a modulated spectrum on the detector 24.
  • IFFT inverse fast Fourier transform
  • modulation frequencies can be obtained from the spectrum, each associated with a distance value.
  • either the spectra of the calibration light and the measurement light must be free from overlap, or the calibration can not be performed simultaneously with the measurement.
  • FIG. 14 schematically shows a part of the spectrometer 20.
  • a dispersive optical element can be seen, which is designed here as a transmission grating 84, primarily for reasons of better representability.
  • the transmission grating 84 is arranged in the collimated beam path, as is the case with the reflection gratings 56 shown in FIGS. 5, 11 and 12.
  • the condenser lens 57 focuses the diffracted light on the detector 24.
  • the detector 24 has not just one, but two pixel rows 86, 88.
  • first pixels 26-1 are arranged, on which only the measuring light 12 'can fall.
  • second pixels 26-2 are arranged, on which only the calibration light 32 can fall.
  • the measuring light 12 ' also falls axially parallel to the dispersive optical element (transmission grating 84) in this exemplary embodiment. Since the diffractive structures of the transmission grating 84 extend along the x direction, the measuring light 12 'is deflected wavelength-dependent in the yz plane and directed by the converging lens 57 to one of the first pixels 26-1 of the first pixel row 86, as with the embodiments described above is the case.
  • the collimated calibration light 32 does not impinge on the transmission grating 84 axis-parallel, but with respect to the xz plane at an angle other than zero.
  • the converging lens 57 does not focus the calibration light 32 diffracted in the yz plane on one pixel 26-1 of the first pixel row 86, but on one of the second pixels 26-2 of the second pixel row 88 arranged in the x direction offset therefrom. Due to the different directions of incidence, the calibration light 32 and the measurement light 12 'can not therefore point to the same pixel be focused when the wavelength and thus the diffraction angle is identical, as is assumed in the figure 14.
  • calibration and measurement can be performed simultaneously, even if the calibration light 32 and the measurement light 12 'have identical spectrums.
  • This approach can therefore be combined particularly well with the exemplary embodiment shown in FIG. 11, in which the calibration light 32 and the measurement light 12 'are generated by the same LED 34 and therefore have identical spectra.
  • both pixel rows 86, 88 are arranged in the same detector 24 and even directly adjacent in the illustrated embodiment, the pixels in the two pixel rows 86, 88 always have identical y positions.
  • the positions of the second pixels 26 - 2 acted upon by the calibration light 32 can be directly closed to the positions of the first pixels 26 - 1 arranged below them. For the evaluation, it thus makes no difference whether the calibration light 32 falls on the first pixels 26-1 or the second pixels 26-2 arranged above them.
  • the calibration light 32 can be guided via its own fibers.
  • the two ends of the fibers are to be arranged side by side.
  • an offset of the fiber ends would be provided along the x-direction.
  • a beam tilting can also be brought about in other ways, eg. B. by the use of wedge prisms.
  • the desired spatial separation of calibration light and measuring light on the detector can be ensured not only by different directions of incidence of the calibration light and the measuring light on the dispersive optical element.
  • suitable polarizing filters which are arranged directly in front of or on the pixels 26-1, 26-2, it can be achieved that calibration light only falls on pixels on which no reflected measuring light with the same wavelength can fall, and vice versa.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Length Measuring Devices By Optical Means (AREA)
  • Spectrometry And Color Measurement (AREA)

Abstract

Un dispositif de mesure destiné à mesurer sans contact une distance à une surface (19) ou une distance entre deux surfaces comprend une source de lumière de mesure (11), qui génère une lumière de mesure polychromatique (12), et une tête de mesure optique (16) qui oriente la lumière de mesure (12), générée par la source de lumière de mesure (11), sur un objet de mesure (18) et reçoit une lumière de mesure (12') réfléchie par celui-ci. Un spectrographe (20) pourvu d'un élément optique dispersif (22) et d'un détecteur (24) analyse spectralement la lumière de mesure réfléchie (12'). Une source de lumière d'étalonnage (30) génère une lumière d'étalonnage (32) et de composition spectrale connue indépendante de la température. À partir de modifications du spectre produit par la lumière d'étalonnage (32) sur des cellules photosensibles (26) du détecteur (24), un dispositif d'évaluation (28) déduit des valeurs de correction avec lesquelles une relation prédéterminée entre les cellules photosensibles (26) d'une part et les longueurs d'onde ou des grandeurs dérivées des longueurs d'onde, d'autre part est modifiée.
PCT/EP2018/075441 2017-09-29 2018-09-20 Procédé et dispositif de mesure sans contact d'une distance à une surface ou d'une distance entre deux surfaces WO2019063403A1 (fr)

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JP2020518006A JP7410853B2 (ja) 2017-09-29 2018-09-20 表面に対する間隔又は２つの表面の間の間隔を非接触で測定する方法と装置

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DE102021124048A1 (de) 2021-09-16 2023-03-16 Precitec Optronik Gmbh Optische Dickenmessvorrichtung
DE102021211046A1 (de) 2021-09-30 2023-03-30 Micro-Epsilon Optronic Gmbh Spektrometer, Abstandsmesssystem sowie Verfahren zum Betreiben eines Spektrometers
DE102022131700A1 (de) * 2022-11-30 2024-06-06 Precitec Optronik Gmbh Optische interferometrische Messvorrichtung und Verfahren

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DE102017122689A1 (de) 2019-04-04

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